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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Photodiodes in photovoltaic mode, speed considerations.
Date: Wed, 16 Oct 2002 00:51:31 GMT
NNTP-Posting-Date: Tue, 15 Oct 2002 17:51:31 PDT
I get a lot of 780 nanometer and 650 nanometer lasers with built in
power monitor diodes for laser power control. There are two types I see.
One type has the cathode of the laser and the cathode of the photodiode
connected together. The second type has the photodiode completely
independent. Looking inside with a microscope, I can see that because
the monitor diode is a separate die, the two types are a bonding and
package difference. The diode specifications I always get are reverse
bias specifications for both types. A systm limitation is that there is
only +5v and +12v available. I cannot generate a negative voltage and
remain inside system design parameters.
The independent photodiode devices are simple to work with. The reverse
biased photodiode is very fast so that laser power can be controlled
with a DAC, counter and comparator scheme (which is part of an IC in the
system) with any counter clock we might use (likely not above 13 MHz).
The common cathode devices, of course, must be used in photovoltaic mode
and the photodiode is as slow as molasses in January. The problem is
that I don't know how slow. That is, I can find a clock frequency for
power control for a particular unit but I can't predict from reverse
characteristics how a photodiode will behave in photovoltaic mode. The
problem exists because I use lasers from more than one manufacturer and
I don't have any control over that.
The problem is predicting the clock to use given a laser from any maker
who delivers the common cathode arrangement. Does anyone know how to
predict the speed of a photodiode from reverse characteristics or
possibly from dimensions which I can measure under a microscope?
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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